Tankless Water Heater Sizing Guide: Get It Right the First Time

Too small = lukewarm disappointment. Too big = wasted money. Here's how to size it right.

I'm Justin Sims, and I've seen a lot of tankless water heater installations go wrong—not because of the unit, but because of sizing.

A tankless water heater that's too small leaves you with lukewarm showers when multiple fixtures run. One that's too big costs more than necessary and doesn't perform any better.

Let me show you how we size tankless units so you get exactly what you need.

How Tankless Sizing Works

Unlike tank water heaters (sized by gallon capacity), tankless units are sized by:

1. **Flow Rate (GPM):** How many gallons per minute the unit can heat
2. **Temperature Rise:** How many degrees it can raise the incoming water temperature

These two factors work together. The colder your incoming water, the harder the unit has to work—which reduces the flow rate it can handle.

Step 1: Identify Your Peak Demand

Think about your household's maximum simultaneous hot water usage. What's the most you'd ever use at once?

Common flow rates:

| Fixture | Flow Rate (GPM) |

|---------|-----------------|

| Shower | 1.5-2.5 |

| Bathroom faucet | 0.5-1.5 |

| Kitchen faucet | 1.0-2.0 |

| Dishwasher | 1.0-2.0 |

| Washing machine | 1.5-3.0 |

Example scenarios:

Morning rush (family of 4):

• 2 showers running = 4.0 GPM
• Kitchen faucet = 1.5 GPM
• Total: 5.5 GPM

Evening usage:

• 1 shower = 2.0 GPM
• Dishwasher = 1.5 GPM
• Total: 3.5 GPM

**Your peak demand = the highest total** you'll realistically run simultaneously.

Step 2: Determine Temperature Rise Needed

Temperature rise = desired hot water temperature - incoming water temperature

Central Texas incoming water temperature:

• Winter: 55-60°F
• Summer: 70-75°F

**Desired hot water temperature:** 110-120°F is typical

Calculation:

• Winter: 120°F - 55°F = 65°F rise
• Summer: 120°F - 75°F = 45°F rise

**Always size for winter conditions** (worst case). That 65°F rise is your planning number.

Step 3: Match Unit Capacity

Now cross-reference your peak GPM with the temperature rise you need.

Here's how common residential tankless units perform at 65°F rise:

| Unit Size (BTU) | GPM at 65°F Rise |

|-----------------|------------------|

| 150,000 BTU | 2.5-3.0 GPM |

| 180,000 BTU | 3.5-4.0 GPM |

| 199,000 BTU | 4.0-5.0 GPM |

| 199,000+ BTU (high efficiency) | 5.0-6.0 GPM |

**Note:** These are approximate. Actual performance varies by brand and model.

Real-World Sizing Examples

Example 1: Couple in a 2-Bathroom Home

**Peak demand:** 1 shower + 1 bathroom faucet = 3.0 GPM

**Temperature rise needed:** 65°F

**Recommendation:** 180,000 BTU unit (comfortable margin)

Example 2: Family of 4 in a 3-Bathroom Home

**Peak demand:** 2 showers simultaneously = 4.0 GPM

**Temperature rise needed:** 65°F

**Recommendation:** 199,000 BTU high-efficiency unit

Example 3: Large Family or Luxury Home

**Peak demand:** 2 showers + kitchen + washing machine = 7.0 GPM

**Temperature rise needed:** 65°F

**Recommendation:** Multiple units or a commercial-grade unit

For very high demand, installing two tankless units is often better than one oversized unit.

Special Considerations for Hill Country Homes

Large Master Suites

Those beautiful rain showerheads and body sprays? They use more GPM than standard fixtures:

• Rain showerhead: 2.0-2.5 GPM
• Body spray (each): 0.5-1.0 GPM

A luxury shower with multiple sprays can use 5+ GPM alone. Factor this in.

Outdoor Kitchens

If you have an outdoor kitchen with a sink or prep area, add that to your calculation.

Pool Houses and Guest Houses

These may need their own tankless units. Running long distances reduces efficiency.

Hot Tub/Spa Fill

Tankless units can fill hot tubs, but it takes time (they're not designed for single large fills). Consider this in your planning.

Gas vs. Electric Tankless

Gas tankless:

• Higher flow rates
• More common for whole-house applications
• Requires gas line sizing verification
• Needs proper venting

Electric tankless:

• Lower flow rates
• Good for point-of-use (single bathroom or kitchen)
• Requires significant electrical capacity
• Simpler installation (no venting)

For most Hill Country homes, gas tankless is the better whole-house solution.

Gas Line Considerations

Most tank water heaters use a 1/2" gas line. Many tankless units require 3/4" or larger.

During your assessment, we check:

• Existing gas line size
• Distance from gas meter
• Other gas appliances on the same line
• BTU capacity available

Upgrading the gas line adds cost but is often necessary for proper performance.

The Importance of Proper Installation

A correctly sized unit, poorly installed, will underperform. Proper installation includes:

• Correct gas line sizing
• Adequate venting (condensing vs. non-condensing)